1. Field of the Invention
The present invention generally relates to a radiation detecting circuit, and, more particularly, to a circuit arrangement for correcting a signal adversely influenced by a photon-detected position on a radiation detector.
2. Description of the Related Art
Various radiation detecting devices have been proposed to acquire radiation data such as photon energies and photon distributions. A direct conversion type semiconductor detector for converting photon energies of radiation directly into electronic signals has been recently widely used for a spectrometer/counter and the like for radiation, e.g., .gamma.-rays.
An operation principle of such a semiconductor .gamma.-ray detector will be described below.
As shown in FIG. 1, when photons 1 are incident on a bulk type semiconductor .gamma.-ray detector 2, photon energies are converted into pairs of electrons 3 and holes 4 by Compton scattering or photoelectric absorption effects. A total sum of energies of the electron-hole pairs generated in semiconductor .gamma.-ray detector 2 is equal to energy Ei of the incident photons. Of the pairs of electrons and holes, electrons 3 move toward high voltage electrode 5 and holes 4 move toward ground electrode 6 by an electric field applied to these electrodes of semiconductor .gamma.-ray detector 2. An induction current is produced by change over time in an electric field (electric charge) induced on the electrode surfaces by migration of electrons 3 and holes 4 in the semiconductor. The induction current is conducted to an external circuit. FIG. 2 illustrates changes in the electric field on both the electrodes caused by movement of electrons 3.
Assuming that electric charges of electrons and holes are respectively "e" and a distance between the electrodes is "D", electric charge "q", induced on the electrode surfaces when electrons 3 move between electrodes 5 and 6 by distance "X" as shown in FIG. 3, is obtained as follows: EQU q=e.multidot.X/D (1)
Therefore, assuming that the total electric charge of electrons (holes) generated by the incident photons is "Q.sub.total ", the total electric charge induced on the electrode surfaces by the electron-hole pairs generated at a position separated from the high voltage electrode surface by distance "X" is: ##EQU1## for Q.sub.e =Q.sub.h. Thus, Q.sub.total is equal to the total electric charge of electrons (or holes) initially generated by the incident photon energies, and a voltage output proportional to the incident energies can be obtained by a charge sensitive converting circuit.
In addition, the induction current flows through the external circuit only while electrons (or holes) move between these electrodes. Induction current duration times "T.sub.reX "0 and "T.sub.rhX ", of the respective electrons and holes are obtained as follows: EQU T.sub.reX =X/V.sub.de EQU T.sub.rhX =(D-X)/V.sub.dh ( 3)
where V.sub.de and V.sub.dh are drift velocities of electrons and holes, respectively. Assuming that .mu.e and .mu.h are respectively mobilities of electrons and holes and E is the electric field, drift velocities V.sub.de and V.sub.dh are obtained as follows: EQU V.sub.de =.mu..sub.e .multidot.E EQU V.sub.dh =.mu..sub.h .multidot.E (4)
In addition, the magnitude of the flowing induction current is: EQU I=I.sub.e +I.sub.h =O.sub.e /T.sub.re +O.sub.h /T.sub.rh ( 5)
where Ie and Ih are respectively currents caused by movement of electrons and holes, and T.sub.re and T.sub.rh are respectively times required for electrons and holes to move between the electrodes and are obtained as follows: EQU T.sub.re =D/V.sub.de, T.sub.rh =D/V.sub.dh ( 6)
FIG. 4 is a graphic representation showing an induction current caused by movement of electrons and holes. Since mobilities of electrons and holes are different from each other, their current values and induction current duration times are different from each other. Note that in FIG. 4, reference numeral 7 denotes an induction current cased by electrons; and 8, an induction current caused by holes.
Integral values of the currents are total electric charges induced on the electrode surfaces. That is: ##EQU2##
As is apparent from the above equation (2), the incident photon energies can be recognized as the total sum of the electric charge induced by movement of electrons and the electric charge caused by movement of holes.
However, in a semiconductor radiation detector, electrons and holes may sometimes be trapped during movement in the detector because of impurities and vacancies moving in the crystal inside the detector. Assuming that a lifetime of electrons (or holes) determined by trapping is .tau..sub.e (or .tau..sub.h), an electric charge induced by movement of electrons and holes is as follows: EQU q=(V.sub.de .multidot.Q.sub.e .multidot..tau..sub.e)/D.multidot.(1-e.sup.X/.tau..sub.e V.sub.de)+(V.sub.dh .multidot.Q.sub.h .multidot..tau..sub.h)/D.multidot.(1-e.sup.- (D-X)/.tau..sub.e V.sub.dh ( 8)
Furthermore, in a room-temperature operating semiconductor detector of CdTe (cadmium telluride) with a high efficiency, mobility of holes is often considerably smaller than that of electrons (i.e., V.sub.dh &lt;V.sub.de), and the lifetime of holes is often considerably smaller than that of electrons (i.e., .tau..sub.h &lt;.tau..sub.e).
Consequently, an electric charge induced on electrode surfaces is mostly caused by movement of electrons, and contribution of holes thereto is small. As a result, the equation (8) is approximated as follows: EQU q=(V.sub.de .multidot.Q.sub.e .multidot.t.sub.e)/D.multidot.(1-e.sup.-X/.tau..sub.e V.sub.de) (9)
Therefore, the total electric charge induced on the electrode surfaces depends on a generating unit (i.e., an incident position of photons) of electron-hole pairs, thereby significantly degrading energy resolution of the detector. For this reason, in the obtained radiation energy distribution, a problem of an unclear photo-peak appears in FIG. 5.
Recently, a gamma camera aiming at a high positional/energy resolution, a high counting efficiency, and compactness has been developed. In this gamma camera, semiconductor .gamma.-ray detectors of the type described above are arranged in a matrix array.
An electronic circuit for extracting an output signal form such a gamma camera is constituted by charge sensitive preamplifier 110 connected to semiconductor .gamma.-ray detector element 100, waveform shaper/amplifier 112, SCA (single channel analyzer) 113 for selecting only photons having proper energies, and counter/memory 114, as shown in FIG. 6. The electronic circuit displays on a display unit (hnow shown) a two-dimensional distribution of a radiation source in real time, or after integrating for a predetermined period of time. Although such electronic circuits must be provided for the same number as that of detectors constituting a camera matrix, only one electronic circuit is shown in FIG. 6 for the sake of simplicity.
However, it is very difficult to arrange detectors and electronic circuits, the total number of which correspond to the number of elements (e.g., m.times.n elements) constituting the matrix array. On the other hand, in order to reduce the total number of electronic circuits, signals may be extracted from both the high voltage and ground potential sides of one detector. That is, as shown in FIG. 7, as for high voltage side 115, outputs are common in a lateral direction (line direction), and as for ground side 116, outputs are common in a transverse direction (column direction), thereby extracting signals. For example, such a conventional signal extracting method is described in "Cadmium telluride matrix gamma camera" Jerry D. Allison, Medical Physics 7(3), May/June 1980, pages 202 to 206, American Association Phys. Med.
However, if such a conventional signal extracting method is employed, detectors must be required to corespond to the number of matrix's elements. Thus, the detectors must be packed at high density to increase positional resolution, that necessarily requires a very difficult and complex manufacture technique.
Therefore, the present invention has been made in consideration of the situation as described above, and has as its primary object to provide a radiation detecting circuit with high energy resolution, which is capable of faithfully recognizing incident energies independently of an incident position of photons incident on a radiation detecting circuit.
In addition, another object of the present invention is to provide a gamma camera which is capable of utilizing such a radiation detecting circuit with high energy resolution, thereby realizing high resolution which a smaller number of detectors.